First of all, there is a difference between nominating candidates and then electing them to the office. Nominating those candidates is not covered in the Constitution except for qualifications such as age and citizenship.
I would like very much to see that non-partisan nominating method Will lists. I suggest 12 or 14 candidates in a national primary where the top four become the candidates in the general election run-off. Meanwhile........
http://avalon.law.yale.edu/subject_menus/debcont.aspThe above minutes of the Convention of 1787 are based on James Madison's notes of the convention. You don't have to be a constitutional scholar to read them and see what the "Founder's intent was regarding any topic.
I have been a constitutional hobbyist for some time and I don't recall much about the presidential candidates' selection. There may be something about factional politics. I'll be looking for whatever might support George Will's assertions or the claim to unconstitutionality of Barrack Obama's election.